Specific Labeling of Intracellular Toxoplasma gondii with Uracil*

Abstract

Radioactive uracil was not significantly incorporated into the nucleic acids of human fibroblast cells. Infection of these cells with T. gondii resulted in an exponential increase in the rate of uracil incorporation that paralleled the exponential growth of the parasite. One day after infection the rate of uracil incorporation was increased 100-fold. Autoradiography showed that all of the [3H] uracil was incorporated into the intracellular parasites. This difference in ability to use uracil may be due to the specific activity of uridine phosphorylase being 100-fold greater in partially purified parasites than in the host cell.